Voltage-controlled exponential oscillator

ABSTRACT

An accurate exponential characteristic is obtained by employing as a diode a silicon carbide lamp having a low temperature coefficient, and by restricting the frequency of the oscillator itself to one octave, the necessary wide range of output frequencies being obtained in a frequency divider. The oscillator is automatically biased to reset it repeatedly to the bottom of its restricted frequency range, as the input continues to rise, such resetting being coordinated with selection of the appropriate frequency range of the frequency divider in order that the output should continue to advance in frequency with the input voltage.

United States Patent r A MP: lF/EBS Inventor David O. Rocheleau Ottawa, Ontario, Canada Appl. No. 23,399 Filed Mar. 27, 1970 Patented Sept. 14, 1971 Assignee Canadian Patents and Development Limited Ottawa, Canada VOLTAGE-CONTROLLED EXPONENTIAL OSCILLATOR 4 Claims, 2 Drawing Figs.

US. Cl 331/61, 84/l.0l, 307/271, 328/142, 331/74, 331/111, 331/ 177 Int. Cl 1103b 3/04, H03k 3/26 Field of Search 331/177, 177 V, 179, 61, 60, 74, 1 1 1;84/l.01, 1.04; 307/271; 328/142, 150

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9 22 /8 656/1 1 A 70A 62/ vs! OTHER REFERENCES Moog, Voltage-controlled Electronic Music Modules, Journal of the Audio Engineering Society, July 1965, Vol. 13, pp. 200- 206. (84- L01) Primary ExaminerRoy Lake Assistant ExaminerSiefrid H. Grimm ABSTRACT: An accurate exponential characteristic is obtained by employing as a diode a silicon carbide lamp having a low temperature coefficient, and by restricting the frequency of the oscillator itself to one octave, the necessary wide range of output frequencies being obtained in a frequency divider. The oscillator is automatically biased to reset it repeatedly to the bottom of its restricted frequency range, as the input continues to rise, such resetting being coordinated with selection of the appropriate'frequency range of the frequency divider in order that the output should continue to advance in frequency with the input voltage.

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VOLTAGE-CONTROLLED EXPONENTIAL OSCILLATOR This invention relates to improvements in voltage-controlled oscillators of the type in which a linearly varying input voltage will produce an exponentially varying output frequency.

There are numerous applications for such an exponential oscillator, for example, as a test oscillator for recording room reverberation curves or equipment response curves, as a test oscillator for generating accurately controlled frequency versus time relationships in the production testing of components such as speaker cones, as a test oscillator for physiological tests, such as hearing aid fittings, or as a function generator generally. One specific purpose for which the oscillator has been developed is for incorporation into an electronic musical instrument. Such devices are now widely used by musicians. The voltage which determines the pitch may be obtained from keyboards, potentiometers, magnetic tapes or computers and a series of different outputs (sine wave, square wave, sawtooth wave, etc.) are provided for use by the musician in achieving various musical effects. The output is required to vary exponentially, since this is the nature of the musical scale.

One of the main problems encountered in the past in such devices has been a tendency for the frequency to drift noticeably with quite small temperature changes (1 or 2), necessitating frequent retuning. Music requires an accuracy of about one part in a thousand. This disadvantage of frequency drift has been due primarily to the relatively high temperature coefficients of diodes commonly used for obtaining the exponential characteristics. Thus the use of diodes in exponential generators for electronic music oscillators is known, see for example Robert A. Moog Voltage-Controlled Electronic Music Modules, Journal of the Audio Engineering Society, July 1965, but the form of diode used has usually been a standard solid state diode. This has had the disadvantage of introducing comparatively large temperature coefficients into a circuit with consequent difficulties in terms of drift of the oscillator frequency.

Another aspect of oscillators designed for the production of music is that a wide frequency range is required, that is at least 4 octaves, and often five or six. Maintenance of an accurate exponential characteristic over such a wide range is a demanding design consideration that may lead either to a need for undesirable complexity, if the error is to be kept low, or altematively to some sacrifice in performance at the ends of the range, in the interests of economy.

The object of the present invention is to provide an oscillator circuit that simultaneously avoids or minimizes both these previous drawbacks by furnishing a circuit that is accurate over a large temperature range (at least l to the oscillator proper having only a limited range, eg a single octave, so as to be able to ensure close observance of the exponential characteristic, while the circuit as a whole still affords the full range of output extending over at least 4 octaves, and more if desired.

To this end the invention consists of an oscillator circuit comprising a. a diode having an exponential voltage/current characteristic and a low-temperature coefficient over a first voltage range,

b. input means connected to the diode for receiving a linearly varying input voltage extending over a second voltage range greater than said first voltage range,

c. detecting means connected to the input means for detecting a voltage therein exceeding said first range,

d. biasing means controlled by the detecting means and connected between the input means and the diode to bias the voltage received by the diode to maintain such voltage within said first range,

e. oscillating means connected to the diode for generating an output having a frequency varying exponentially with the voltage received by the diode, such output extending over a first frequency range corresponding to said first voltage range,

f. frequency divider means connected to receive said output for dividing the same to generate oscillations extending over a second frequency range greater than said first frequency range and corresponding to said second voltage range,

g. output means connected to the frequency divider means for receiving oscillations therefrom within a selected portion of said second frequency range, and

h. means connecting the detecting means to the output means to select said portion in accordance with the bias applied to the diode.

Use of a silicon carbide lamp as the diode introduces into this type of circuit a device that has a very low-temperature coefficient compared to other conventionally used diodes. By limiting the operation of the oscillating means to the so-called first frequency range (e.g. l octave) the purity of the exponential characteristics can be more readily retained, since this limited range can conveniently correspond to a similarly limited voltage range, i.e. the so-called first voltage range (e.g. 1 volt). When the input voltage exceeds 1 volt, the detecting means (e.g. in the form of a differential comparator) detects this fact and causes the biasing means (eg voltage source and octave switch) effectively to reset the oscillating means input to zero. Thus a true input voltage of say 1.2 volts would appear at the oscillating means as 0.2 volts. At the same time the detecting means will select an output from the frequency divider means in a frequency range above the first one, e.g. in a second octave. In a similar fashion, an input voltage above 2 volts can be detected by the detecting means to bias the oscillating means by 2 volts to keep its actual input below 1 volt, at the same time selecting the next higher frequency range at the output, e.g. a third octave.

Other features of the preferred form of the invention will be apparent from the specific circuit that follows, which circuit has been provided by way of example only and not by way of limitation of the broad scope of invention, which scope is defined in the appended claims. In the drawings:

FIG. 1 is said specific circuit; and

FIG. 2 is pulse diagram.

The input is assumed to be an instrument keyboard (not shown), although as explained above, other voltage inputs may be employed. The keyboard switches energize one of a series of input terminals, of which terminals 10, I1 and 12 have been shown as representative. The DC voltage from such an energized terminal 10, for example, together with a bias from source 13, is applied to the input of a unity gain input amplifier 14, whose output feeds to an oscillator driver amplifier 15 and a comparator driver amplifier 16. The amplifier l5 energizes oscillating means in the form of a relaxation oscillator 17 comprising a complementary unijunction transistor 18, resistor 19, capacitor 20, output transistor 21, and silicon carbide lamp 22. Thus lamp 22 acts as a diode and consequently has an exponential voltage/current characteristic, at least over a limited range.

The oscillator 17 is a l-octave, exponential, relaxation oscillator. The use of a complementary unijunction transistor (for example the transistor sold by General Electric of Syracuse, N.Y., under the designation DSKl or alternatively, an equivalent pair of complementary transistors, in combination with the silicon carbide lamp, provides excellent frequency stability with temperature, while still affording the exponential characteristics required for music. Since the exponential range is limited, however, the oscillator is limited to l octave, the necessary additional octaves being obtained in the manner described below.

For example, the circuit can conveniently be so set up that a change from a 0 to l volt at the input terminals will change the frequency by l octave. The oscillator can be set so that 33,488 Hz. corresponds to 0 input volts and 66,976 Hz. corresponds to 1 input volt. This small input range (1 volt), amplified approximately 8 times in the oscillator driver amplifier 15 (the amplifier 14 having unity gain), still enables the silicon carbide lamp to be operated solely within its truly exponential range. if the voltage applied by the keyboard to the input terminals will vary linearly from to 1, the corresponding oscillator frequency will vary exponentially from 33,488 to 66,976 Hz.

The output from the oscillator 27 on lead 23 is a series of pulses 24, as shown in FIG. 2. These pulses 24 enter a frequency divider circuit 25 in the form of a ripple through counter comprising five gated flip flop stages 26 to 30. The first stage 26 is turned on by the trailing edge of one pulse 24 and then off by the next, to produce at its output a train of pulses 31, having half the frequency of the pulses 24. The second stage 27 is similarly turned on and off by the trailing edges of the pulses 31 to generate a quarter frequency train of pulses 32. Similarly, the stage 28 will generate pulses 33 of one-eighth the basic frequency; and stage 29 pulses 34 of one-sixteenth. Thus, if the basic frequency is 33,488 1-12., the output from stage 29 will be 2,093 Hz. which musically is the note C. The final stage 39 will commence to act similarly but will be prevented from doing so by its own output which is connected through an OR gate 40 to a one shot multivibrator 41, the output of which is a single pulse 42 of predetermined length, which pulse is fed back to the reset inputs of all the stages of the frequency divider circuit 25. Thus the pulse 35 is cancelled almost as soon as it commences (after a short delay in the circuit), as indeed are the simultaneously appearing pulses of all the stages. in fact, at this moment all the other stages are between pulses. The reset pulse 42 is short lived, the circuit 25 immediately recommencing operation as before, so that the next pulse 35 will occur as shown in FIG. 2 with the same frequency as that of the pulses 34. Thus the last stage 30 only divides the frequency by one, and the output received by an octave divider 43, namely the pulses 35, are at one-sixteenth of the oscillator frequency.

The outputs of stages 26 to 29 are fed as inputs to respective lNl-llBlT gates 36 to 39, for a purpose later to be described. Under the conditions just described, all the gates 36 to 39 are inhibited, so that no other pulses reaclt the OR gate 40.

As has been explained above, a change of the input voltage received from the keyboard from O to 1 volt will result in the oscillator frequency moving 1 octave from 33,488 to 66,976 Hz., with the output from stage 31) (pulses 35) varying correspondingly from 2,093 to 4,186 Hz. Since it is desired to operate the oscillating means only over a single octave, in order to maintain accurately its exponential characteristics, it is necessary to reset the oscillator to its lower value to accommodate the next octave, as soon as the input voltage rises above 1 volt. When this happens, the output of the comparator driver amplifier 16, which has unity gain, will go slightly more positive than 1 volt. This amplifier is connected to detecting means in the form of four differential comparators 46 and 49, each of which also has as an input a reference source, these sources taking the form of adjustable resistors 56 to 59 and amplifiers 66 to 69. The outputs of the amplifiers 66 to 69 are adjusted to be 4, 3, 2 and 1 volts respectively. Thus when the output of the comparator driver amplifier 16 rises above 1 volt, the differential amplifier 49 detects the fact that the value of its input derived from the amplifier 16 exceeds the fixed value of its input from the amplifier 69, causing the amplifier 49 to emit an output to change the state of a flip-flop 79, eg from logic 1 to logic 0.

The output of the flip-flop 79 is connected to a first octave switch 89, the logic 0 closing this switch to apply 1 volt DC to a lead 90 which is connected to an input of the oscillator driver amplifier 15. The latter being a summing amplifier gives an output representative of the amount that its input received from the amplifier 14 exceeds that from the lead 90. Thus the oscillator driver amplifier 15 has effectively been biased or reset to operate from a base of +1 volt, so that the oscillator 17 continues to oscillate within its basic octave of 33,488 to 66,976 Hz. As soon as the output from the amplifier 16 exceeds 2 volts, it actuates the differential amplifier 48 to change the state of a flip-flop 78 to close a second octave switch 88 which applies 2 volts to the second input of the oscillator driver amplifier 15 to bias it to operate from a base of +2 volts. Thus the oscillator 17 again comes back to the lower end of its octave. Similarly, when the output of the amplifier exceeds 3 and 4 volts, respectively, the differential amplifiers 47 and 48 each change the state of a flip-flop 77 or 76 to close a third or fourth octave switch 87 or 88 to apply 3 volts or -4 volts to the amplifier 15. The negative voltage sources and the octave switches thus constitute biasing means. Obviously, additional octave stages can be added, if desired.

While the oscillator 17 has to be brought back to its starting frequency each time an octave change was made, the frequency of the output has been doubled. For this purpose the output of the flip-flop 79 is connected as a second input to the IN- HIBlT gate 39. The logic 1 previously received from all the flip-flops 76 to 79 will have kept the gates 36 to 39 inhibited, but now the gate 39 is enabled by the logic 0 received from the flip-flop 79 when the system moves up to the second octave condition. With gate 39 enabled, pulses 34 from stage 29 pass through gate 40 to actuate the multivibrator 41, the output pulses 42' of which act as reset pulses on stage 29 in the same manner as previously explained in relation to stage 30. Thus stage 29 is now prevented from dividing by two; instead it divides by one, and the effect is that the pulses 34' and 42' now have a frequency of one-eighth the oscillator frequency, or 1 octave above that of pulses 35 and 42.

[n a similar fashion, when the gate 38 is enabled by the flipflop 78 for the third octave, the frequency halving effect of stage 28 is blocked by the reset operation so that its output and hence that of the multivibrator 41 are now at one-quarter the oscillator frequency. As gates 37 and 36 are successively enabled, the output frequency is likewise raised first to onehalf oscillator frequency and finally to full oscillator frequen- The output from the multivibrator 41, that is the pulses 42, or 42', or the higher frequency equivalents thereof, passes to the 6 octave divider 43 which is a conventional frequency dividing network that furnishes square wave outputs at a half, a quarter, one-eighth, one-sixteenth, one thirty-second, and one sixty-fourth of the input frequency f. Thus assuming the input from the multivibrator 41 to be the basic value of 2,093 Hz., the divider 43 will furnish frequencies down to and even below the bottom of the audible range, the frequency f/8 being 261.625, which is Middle C. A conventional staircasing network 44 reshapes the square pulses received from the divider 43 by the mixing of frequencies f/2, f/4, f/8 and f/l6 to form pulses with sharp leading edges and stepped trailing edges. The output of the network 44 passes through a recovery amplifier 50 to smooth out the steps and provide a sawtooth output at terminal 51. Square pulses direct from the divider 43 are supplied to a triangular integrator 52 to yield a triangular waveform output at terminal 53. The triangular pulses are also taken through a sine integrator 54 to yield a sinewave output at terminal 55. The pulses from the divider 43 enter a variable pulse width amplifier 60 to yield a pulsed output of controlled pulse length (adjustable from 10 p. sec. to 10 msec.) at terminal 61, and also enter a recovery amplifier 62 to yield at terminal 63 a square wave output, i.e. relatively long pulses. An octave switch 64 is connected in the outputs of the staircasing network 44 and of the divider 43, and is used to change the output frequencies manually by 4 octaves. The nature of the outputs at terminals 51, 53, 55, 61 and 63 are conventionally those required in electronic musical instruments, and indeed this entire portion of the circuit illustrated in FIG. 1, the divider 43 and downstream therefrom, represents a conventional set of so-called tone color resources".

1 claim:

1. An oscillator circuit comprising a. a diode having an exponential voltage-current characteristic and a low-temperature coefficient over a first voltage range,

input means connected to the diode for receiving a linearly varying input voltage extending over a second voltage range greater than said first voltage range,

c. detecting means connected to the input means for detecting a voltage therein exceeding said first range,

d. biasing means controlled by the detecting means and connected between the input means and the diode to bias the voltage received by the diode to maintain such voltage within said first range,

oscillating means connected to the diode for generating an output having a frequency varying exponentially with the voltage received by the diode, such output extending over a first frequency range corresponding to said first voltage range,

f. frequency divider means connected to receive said output for dividing the same to generate oscillations extending over a second frequency range greater than said first frequency range and corresponding to said second voltage range,

g. output means connected to the frequency divider means for receiving oscillations therefrom within a selected portion of said second frequency range, and

h. means connecting the detecting means to the output means to select said portion in accordance with the bias applied to the diode.

2. An oscillator circuit according to claim 1, wherein said diode is a silicon carbide lamp.

3. An oscillator circuit according to claim 2, wherein said first frequency range is a single octave and said second frequency range is at least 2 octaves.

4. An oscillator circuit according to claim 3,

. wherein said frequency divider means (f) comprises a plurality of flip-flop stages, an input of a first of which is connected to said output of the oscillating means, an input of each successive stage being connected to an output of the preceding stage, whereby each successive stage normally oscillates at a half the frequency of the preceding stage,

j. and wherein said output means (g) comprises 

1. An oscillator circuit comprising a. a diode having an exponential voltage-current characteristic and a low-temperature coefficient over a first voltage range, input means connected to the diode for receiving a linearly varying input voltage extending over a second voltage range greater than said first voltage range, c. detecting means connected to the input means for detecting a voltage therein exceeding said first range, d. biasing means controlled by the detecting means and connected between the input means and the diode to bias the voltage received by the diode to maintain such voltage within said first range, e. oscillating means connected to the diode for generating an output having a frequency varying exponentially with the voltage received by the diode, such output extending over a first frequency range corresponding to said first voltage range, f. frequency divider means connected to receive said output for dividing the same to generate oscillations extending over a second frequency range greater than said first frequency range and corresponding to said second voltage range, g. output means connected to the frequency divider means for receiving oscillations therefrom within a selected portion of said second frequency range, and h. means connecting the detecting means to the output means to select said portion in accordance with the bias applied to the diode.
 2. An oscillator circuit according to claim 1, wherein said diode is a silicon carbide lamp.
 3. An oscillator circuit according to claim 2, wherein said first frequency range is a single octave and said second frequency range is at least 2 octaves.
 4. An oscillator circuit according to claim 3, i. wherein said frequency divider means (f) comprises a plurality of flip-flop stages, an input of a first of which is connected to said output of the oscillating means, an input of each successive stage being connected to an output of the preceding stage, whereby each successive stage normally oscillates at a half the frequency of the preceding stage, j. and wherein said output means (g) comprises i. an OR gate connected to receive an output from each stage, ii. an INHIBIT gate connected between the OR gate and the output of each stage except the last, iii. a multivibrator connected to receive the output of the OR gate, iv. a reset connection extending from the multivibrator to each stage to check oscillation thereof, k. said means (h) connecting the detecting means to the INHIBIT gates for enabling at least one such gate to select the highest frequency received by the multivibrator. 